Modern Control Engineering

Section 4–3 / Pneumatic Systems 115

(a) (b)

(^00) X
X
Ps
Ps
Pb
X
Ps
Pb
Pa
Pc
Pa
Pc

The effect of the feedback bellows is thus to reduce the sensitivity of the controller. The

principle of feedback is commonly used to obtain wide proportional-band controllers.

Pneumatic controllers that do not have feedback mechanisms [which means that

one end of the flapper is fixed, as shown in Figure 4–9(a)] have high sensitivity and are

calledpneumatic two-position controllersorpneumatic on–off controllers. In such a con-

troller, only a small motion between the nozzle and the flapper is required to give a

complete change from the maximum to the minimum control pressure. The curves re-

lating to Xand to Xare shown in Figure 4–9(b). Notice that a small change in X

can cause a large change in which causes the diaphragm valve to be completely open

or completely closed.

Pneumatic Proportional Controllers (Force-Balance Type). Figure 4–10 shows

a schematic diagram of a force-balance type pneumatic proportional controller. Force-

balance type controllers are in extensive use in industry. Such controllers are called stack

controllers. The basic principle of operation does not differ from that of the force-distance

type controller. The main advantage of the force-balance type controller is that it elimi-

nates many mechanical linkages and pivot joints, thereby reducing the effects of friction.

In what follows, we shall consider the principle of the force-balance type controller.

In the controller shown in Figure 4–10, the reference input pressure and the output

pressure are fed to large diaphragm chambers. Note that a force-balance type pneu-

matic controller operates only on pressure signals. Therefore, it is necessary to convert

the reference input and system output to corresponding pressure signals.

Po

Pr

Pb ,

Pb Pc

Output

pressure

Pr

Po

A 1

A 1

A 2

Reference

input pressure

X + x Pc + pc

Atmosphere

Air supply pressureControl

P 1 = k(Pc + pc)

Figure 4–10
Schematic diagram
of a force-balance
type pneumatic
proportional
controller.

Figure 4–9

(a) Pneumatic controller without a feedback mechanism; (b) curves Pbversus Xand versus Pc X.